Many plants, including apple and peach trees and grape vines, naturally set more fruits than is desirable for commercial fruit production. A high fruit crop load can stress the plants which increases their susceptibility to cold injury and disease. Further, having high fruit crop load (set) can cause the plants to only produce fruit every other year, known as biennial bearing.
In addition, the natural high set rates of these plants lead to small fruits which are not as marketable as large fruits. Also high fruit sets may negatively impact the color, sugars, and flavors of the fruits. High fruit sets can also contribute to lower tolerances to storage conditions.
Growers overcome this issue by thinning the fruits which allows the plants to devote their resources to the remaining fruit, thereby producing healthier, larger sized fruits. Thinning can be done by hand or by chemical treatments.
Hand thinning is very resource intensive as it requires someone to remove each undesired fruitlet from the plant. This often requires a lot of time and in the case of trees, the fruitlets may be difficult to reach. Accordingly, hand thinning is very expensive for fruit growers.
Chemical treatments have been used to thin apples, peaches, and grapes. Current commercially available active ingredients used for apple thinning include 1-naphthyl methylcarbamate (Sevin® available from and a registered trademark of Bayer), 6-benzyladenine (MaxCel® available from and a registered trademark of Valent BioSciences Corporation), and 6-benzyladenine with gibberellin 4/7 (Promalin® available from and a registered trademark of Valent BioSciences Corporation). However, each of these commercially available products has their drawbacks. For example, 1-naphthyl methylcarbamate (Sevin®) is a very effective thinner, but it has significant ecotoxicity and bee toxicity. Sevin® has been banned as a thinning agent in Europe. MaxCel® is not as robust of a thinner as Sevin®, but it increases fruit size. Promalin® has some thinning effect, but it is most useful as a fruit shape modification product.
Currently, there is not a commercially available chemical thinner for peach trees. Growers have tried numerous chemical treatments but the results are not consistent or negative side effects make them unusable.
The chemical treatment standard for thinning seedless grapes is gibberellic acid. However, gibberellic acid treatments may cause reduced bloom density in the following year, especially on seeded grapes.
(S)-abscisic acid (“S-ABA”) can thin fruits; however, the effective rates are cost prohibitive.
Accordingly, there is a need for new methods for thinning apple trees, peach trees, and grape vines to produce highly marketable fruits. The new methods should be cost-effective for the growers and produce consistent and reliable thinning results.